A seamless pipe or tube is produced by piercing and rolling a solid round billet by a piercing mill. The piercing mill includes a plurality of inclined rolls and a plug is provided between the plurality of inclined rolls. A pusher device is provided on the inlet side of the piercing mill.
A billet heated in a heating furnace has its rear end pushed by the pusher device and is transported toward between the inclined rolls. When the billet is bitten between the inclined rolls, the pusher stops pushing the billet. The billet engaged between the inclined rolls is pierced and rolled and made into a hollow shell as it is helically rotated.
In the above-described piercing and rolling, leaf-, fin-, or lap-shaped defects (hereinafter referred to as “inner surface defects”) are generated on the inner surface of the hollow shell after the piercing and rolling because of the rotary forging effect and shear deformation.
In order to restrain such inner surface defects from being generated during the piercing and rolling, the piercing and rolling may be carried out with a smaller rolling reduction than in the conventional case. However, if the rolling reduction is reduced, the billet is less stably bitten between the inclined rolls, in other words, so-called defective biting is more likely to result.
A technique for reducing such defective biting is disclosed by JP 2000-246311 A and JP 2001-162306 A. According to the disclosure of these documents, during the period between when the tip end of a billet contacts the inclined rolls and when the billet is bitten between the inclined rolls and stably pierced, the pusher device continues to push the billet from behind. In this way, the defective biting can be restrained. Hereinafter, such piercing and rolling will be referred to as “pusher piercing and rolling.”
The pusher piercing and rolling can indeed restrain the defective biting. However, when the pusher piercing and rolling is carried out, the hollow shell can have wall thickness deviation in some cases. Wall thickness deviation is particularly likely to happen at the tip end of the hollow shell that is pierced and rolled while being pushed by the pusher device.